Simple Factory Pattern

A Simple Factory Pattern returns an instance of one of the several classes depending upon the parameters passed to the shared/non-shared factory method.

Typically, all these classes are inherited from a common parent class. Each of them has common methods. But each of them has their own optimization task operation on data.  

The following code implements the Simple Factory Pattern for the School Login System.

This application allows multi-access level user to login into the school system. Depending upon their userid/password they will be instantiated. 

SchoolPrincipal and SchoolTeacher are the classes derived from the SchoolUser base class. There is a UserFactory class which is responsible to instantiate the appropriate class instance (SchoolPrincipal or SchoolTeacher) depending upon the userid/password passed to its shared method GetSchoolUser. 

This method is entirely responsible for returning the appropriate class instance no matter how complex logic is involved. 

Diagram:

Base Class: 

Public Class SchoolUser
    Public _Fname As String
    Public _Lname As String
    Public _UserType As String

    Sub New() 

    End Sub 

End Class 

Derived Classes:           

            1. This class is for School Principal

            Public Class SchoolPrincipal
                  Inherits SchoolUser
            Sub New()
            _Fname = "David "
            _Lname = "Smith "
            _UserType = "Principal"
            End Sub
End Class

           2. This class is for SchoolTeacher

            Public Class SchoolTeacher
                  Inherits SchoolUser
            Sub New()
                  _Fname = "Patrecia"
                  _Lname = "Terry"
                  _UserType = "Teacher"
            End Sub
End Class 

Factory Class: 

Public Class UserFactory
Shared Function GetSchoolUser(ByVal _login As String,_
ByVal _paswrd As String) As SchoolUser 

            If _login = "principal" And _paswrd = "principal" Then
                  Return New SchoolPrincipal()
            ElseIf _login = "teacher" And _paswrd = "teacher" Then
                  Return New SchoolTeacher()
            Else
                  Throw New Exception()
            End If
            End Function
End Class 

Client application using the factory: 

      Private Sub Button1_Click(ByVal sender As System.Object,_
                  ByVal e As System.EventArgs) Handles Button1.Click
Dim m_OUser As SchoolUser = UserFactory.GetSchoolUser(TextBox1.Text, TextBox2.Text)
MsgBox("User Type: " & m_OUser._UserType & vbNewLine & "Name: " & m_OUser._Fname & " " & m_OUser._Lname)
      End Sub